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2.1.1 Basic concepts of crystal structure (1) Crystal and amorphous All solid substances can be divided into two types: crystal and amorphous according to the arrangement of their atoms. A crystal refers to an object in which atoms (or molecules) are periodically arranged according to certain geometric rules. Figure 2-1(a) shows an example of the simplest body-centered cubic crystal structure. Amorphous refers to objects in which atoms (or molecules) are piled together irregularly. Substances such as rosin, glass, asphalt, etc. are amorphous. In nature, except for a few substances that are amorphous, most solid substances are crystals. Metals are usually crystals in their solid state. The crystal has a fixed melting point and its properties are anisotropic. ； Amorphous crystals, on the other hand, have no fixed melting point and are isotropic. As the temperature increases, amorphous materials will gradually soften and eventually become liquids with significant fluidity. (2) Crystal lattice, crystal packet and lattice constant ○1 Crystal lattice In order to facilitate the understanding of the rules of atomic arrangement inside the crystal, each atom is regarded as a fixed rigid ball, and the crystal is formed by the regular accumulation of these rigid balls. In order to facilitate the analysis of the arrangement and form of atoms in various crystals, some geometric lines are commonly used to connect the centers of each atom in the crystal lattice to form a space grid. The intersection of each connection line is called a node. The small circle (or black dot) on the node represents the center position of each atom. This space lattice, which represents the regular arrangement of atoms in a certain order in a crystal, is called a crystal lattice. As shown in Figure 2-1(b). ○2 Crystal Packet Since the atoms in the crystal are regularly arranged and have the characteristics of repeated arrangement, the crystal lattice can be considered to be composed of many basic geometric objects with the same size, shape and orientation repeatedly stacked in space. This smallest geometric unit that can completely reflect the characteristics of the crystal lattice is called a unit cell, as shown in Figure 2-1(c). ○3 Lattice constant The dimensions a, b, and c of each edge of the crystal packet are called lattice constants. The size is commonly measured in Å (angstrom) (1Å=10-10m). The angle between the edges of the crystal packet is expressed by α, β, and γ respectively. As shown in Figure 2-1(c), the crystal packet with its lattice constant a=b=c, and α=β=γ=90o is called a simple cubic crystal packet, and the lattice with a simple cubic crystal packet is called a simple cubic lattice. (3) Crystal planes and orientations The plane composed of a series of atoms in a crystal is called a crystal plane. A straight line passing through the centers of two or more atoms can represent a certain direction of the spatial arrangement of the crystal lattice, which is called the crystal orientation. In the same crystal, the density of atomic arrangement in different crystal planes and crystal directions is different, and the atomic bonding force is also different. Therefore, the crystal is anisotropic. 2.1.2 Common metal lattice There are many types of metal lattice, but the three most commonly used types are body-centered cubic lattice, face-centered cubic lattice, and hexagonal close-packed lattice. (1) The body-centered cubic lattice is shown in Figure 2-2. The unit cell of the body-centered cubic lattice is a cube, with one atom in the center and eight corners of the cube. The atoms at the top corners of the unit cell are shared by eight adjacent unit cells. Therefore, the number of atoms in a body-centered cubic unit cell is 1/8×8+1=2. Metals with body-centered cubic lattice include alpha iron, chromium (Cr), vanadium (V), tungsten (W) and molybdenum (Mo). (2) The face-centered cubic lattice is shown in Figure 2-3. The unit cell of the face-centered cubic lattice is also a cube, with one atom arranged at the eight vertices of the unit cell and at the center of the six faces. The atoms at the vertex corners of the unit cell are shared by eight adjacent unit cells, and the atoms at the center of each face are shared by two unit cells. Therefore, the number of atoms in a face-centered cubic unit cell is 1/8×8+1/2×6=4. Metals with face-centered cubic lattice include aluminum (Al), copper (Cu), lead (Pb), nickel (Ni) and γ iron (pure iron between 9120 C-13940 C). (3) Hexagonal close-packed lattice is shown in Figure 2-4. The unit cell of hexagonal close-packed lattice is a hexagonal cylinder, consisting of six rectangular sides and two hexagonal faces. One atom is arranged at each of the twelve vertices of the unit cell and at the center of the upper and lower hexagonal faces. At the same time, there are three atoms between the upper and lower hexagonal faces. The atoms at the vertex corners of the unit cell are shared by the six adjacent unit cells, while the atoms at the centers of the upper and lower hexagonal faces are shared by the two unit cells. Therefore, the number of atoms in a close-packed hexagonal unit cell is 1/6×12+1/2×2+3=6. Metals with close-packed hexagonal lattice include beryllium (Be), magnesium (Mg) and zinc (Zn). It can be seen from calculations that among the above three lattice, in terms of the tightness of atomic packing, the body-centered cubic lattice is less compact, while the face-centered cubic lattice and the hexagonal close-packed lattice have the same compactness and are larger. Therefore, when changing from one lattice to another, there will be a change in volume and compactness. 2.1.3 Actual crystal structure of metals (1) A crystal whose internal lattice orientation (i.e. the arrangement direction of atoms) of a polycrystalline crystal is completely consistent is called a single crystal. But among industrial materials, single-crystal metal materials basically do not exist unless they are specially made. Actual metal materials, even in a small piece, contain many small crystals. Even if the internal orientations of each small crystal are the same, the orientations between each small crystal are different. As shown in Figure 2-5, the shape of each small crystal is mostly irregular and granular. This small crystal with irregular shape and polyhedral granular shape is called a grain, and the interface between the grains is called a grain boundary. Obviously, the orientation of the atomic arrangement within the same grain is consistent. ； The orientation of the atomic arrangement is different between different grains. Such crystals composed of many crystal grains are called polycrystals. It can be seen that the actual crystal structure of metal is a polycrystalline structure. Precisely because the atoms of each grain in a polycrystal are arranged in different directions, the anisotropies of the grains cancel each other out, making the mechanical properties of the polycrystalline material isotropic. (2) Crystal defects The actual crystal structure of metal is not only a polycrystalline structure, but also has various crystal defects inside it. Crystal defects refer to areas where the arrangement of atoms within the crystal is disturbed and does not follow the rules of an ideal crystal. The existence of crystal defects will have a significant impact on the properties and structural transformation of metals. According to the geometric characteristics of crystal defects, they can be divided into three categories: point defects, line defects, and surface defects. ○1 Point Defects Point defects refer to defects that are small in length, width, and height. The most common point defects are lattice vacancies and interstitial atoms. As shown in Figure 2-6, point defects are formed when some atoms in the crystal lattice leave the nodes (vacancies) of their crystal lattice due to some reasons (accidental deviations in thermal vibration, etc.) and are in the lattice gap (interstitial atoms). The existence of point defects provides a path for atomic diffusion at high temperatures, which is extremely important for the heat treatment process of metal materials. ； The crystal lattice is distorted at room temperature, that is, the lattice shrinks in the vacancies and the atoms in the gaps expand, which has a certain impact on the crystal properties, such as increasing the strength, hardness and resistance of the material, and reducing the plasticity and toughness of the material. ○2-line defects are defects distributed linearly in the crystal, also known as dislocation lines, or dislocations for short. They are the result of local slippage of one part of the crystal relative to another part of the crystal in the lattice. The intersection line between the slipped part and the unslipped part of the crystal is a line defect. There are many types of line defects, the simplest one is edge dislocation. As shown in Figure 2-7, there is an extra atomic plane arranged along the EF line on the ABCD crystal plane, which inserts into the crystal like a knife edge, preventing the upper and lower atomic planes from being aligned. Therefore, this misalignment of atomic planes is called an edge dislocation. EF lines are called dislocation lines. In the region near the dislocation lines, the crystal lattice is distorted, affecting the properties of the metal. For example, the plastic deformation of metal materials is related to the movement of dislocations. After cold deformation processing, the strength of the metal increases (work hardening), which is caused by the increase in dislocations. ○3-surface defects There are two types of surface defects: grain boundaries and sub-grain boundaries, as shown in Figure 2-8. Grain boundaries refer to the interfaces formed between grains due to different orientations of grains in metal. Even within a grain, the orientation of the atomic arrangement is not completely consistent. It is still composed of many crystal blocks with small orientation differences. Such small crystal blocks are called sub-grains, and the junctions between sub-grains are called sub-grain boundaries. The lattice at the grain boundaries and sub-grain boundaries is in a state of distortion, the energy is higher than inside the grains, and the strength and hardness are higher at room temperature. The more grain boundaries and sub-grain boundaries there are, the greater the dislocation density, and the higher the metal strength and hardness. 2.2 Structure of alloys 2.2.1 Overview Although pure metals have excellent electrical conductivity, thermal conductivity, chemical stability and beautiful metallic luster, almost all pure metals have poor mechanical properties such as strength, hardness and wear resistance, so they are not suitable for making various mechanical parts and tools and molds that require higher mechanical properties. At the same time, the types and performance levels of pure metals are limited, but people's requirements for metal materials are unlimited. Relying only on pure metals cannot meet people's diverse and increasing requirements for metal materials. For this reason, people have been producing and using various alloy materials since ancient times. Alloy refers to a substance with metallic properties composed of two or more metallic elements or metallic elements and non-metallic elements. The most basic, independent units (elements or stable compounds) that make up an alloy are called components. According to the number of components, alloys can be divided into binary alloys, ternary alloys and multi-component alloys. For example, iron-carbon alloy is a binary alloy composed of iron and carbon. The properties of an alloy are determined by its structure, which is composed of phases. The so-called phase refers to the homogeneous parts in a metal or alloy that have the same chemical composition and the same structure and are separated by interfaces. According to the different basic properties of the crystal structure, the phases in solid alloys can be divided into solid solutions and metal compounds. The so-called structure generally refers to the overall body composed of one or more phases with different shapes, sizes and distribution methods that can be seen using metallographic observation methods. A structure composed of only one phase is called a single-phase structure. ； A structure composed of several phases is called a multiphase structure. 2.2.2 Structure of alloy The structure of alloy can be composed of a single phase or two or more basic phases. According to the different interactions between components in the alloy, the structure of solid alloys can be divided into three categories: solid solution, metal compounds and mixtures. (1) Solid solution Solid solution refers to the uniform solid phase formed by the mutual dissolution of alloy components in the solid state. The component in the solid solution that maintains the original lattice structure is called the solvent, and its content is relatively large. ； Other components that are dissolved and the lattice structure disappears are called solutes, and their content is smaller. Solid solution is a basic phase structure of the alloy, and its crystal lattice is the same as the solvent component crystal lattice. According to the different positions occupied by solute atoms in the solvent crystal lattice, they can be divided into two categories: interstitial solid solutions and substitutional solid solutions. ○1 Interstitial solid solution A solid solution formed by solute atoms in the gaps between solvent atoms is called an interstitial solid solution, as shown in Figure 2-9(a). Since the solvent lattice gaps are limited, the number of solute atoms that the interstitial solid solution can dissolve is also limited. Since the solvent lattice gap size is very small, the solute atoms that can form an interstitial solid solution are usually some non-metallic elements with a small radius, such as carbon, nitrogen, boron and other non-metallic elements dissolved in iron to form a solid solution. ○2 Substitution solid solution The solid solution formed by replacing some atoms on the solvent lattice nodes with solute atoms is called a substitution solid solution, as shown in Figure 2-9(b). In substitutional solid solutions, the solubility of a solute in a solvent mainly depends on the atomic radius, electrochemical properties and crystal lattice type of both. Generally speaking, if the two have the same lattice type, similar electrochemical properties, and a small difference in atomic radius, the solubility will be greater. As shown in Figure 2-10, in a solid solution, solute atoms dissolve into the solvent lattice, causing distortion of the solid solution's lattice, thereby increasing the plastic deformation resistance and improving the strength and hardness of the metal material. This phenomenon of increasing the strength and hardness of metal materials by dissolving solute elements to form a solid solution is called solid solution strengthening. Solid solution strengthening is one of the important ways to improve the mechanical properties of metal materials. Practice has shown that proper control of the solute content in solid solution can significantly improve the strength and hardness of metal materials while still maintaining good plasticity and toughness. Therefore, metal materials that require higher comprehensive mechanical properties are all alloys with solid solution as the matrix. (2) Metal compound Metal compound refers to the interaction between alloy components to form a substance with metallic properties, which can generally be expressed by a chemical molecular formula. The lattice type of metal compounds is completely different from the lattice type of the components forming the compound. It has a complex lattice form and is another basic phase structure of the alloy. For example, in iron-carbon alloys, the lattice structure of Fe3C, a metal compound formed by iron and carbon, is shown in Figure 2-11. It is different from the lattice of iron and carbon, but a complex orthorhombic lattice. The properties of metal compounds are different from those of any component. Their melting points are generally higher, hard and brittle, and are rarely used alone. When the alloy contains metal compounds, the strength, hardness and wear resistance of the alloy will be significantly improved, while the plasticity and toughness will decrease. Metal compounds are important constituent phases of many alloy steels, non-ferrous metals and cemented carbide. (3) Mixture A mixture refers to two or more substances composed of a certain mass fraction. The mixture can be composed of two or more solid solutions, or it can be composed of a solid solution and a metal compound (mostly). Each component in the mixture still maintains its original crystal lattice, and its properties depend on the properties of each component phase, as well as their quantity, size, shape and distribution. 2.3 Metal crystallization Metal materials are obtained through the process of smelting and casting solidification. Obtain the required chemical composition of metal materials through smelting ； A solid metal with a crystalline structure is obtained by solidification through casting. Therefore, the crystal structure of metals is formed during the transition from liquid to solid state. The crystallization of metal refers to the process of metal changing from liquid state to solid state, that is, the atoms transition from an irregularly arranged amorphous state to a crystalline state. Studying the crystallization rules of crystals is of great significance for exploring ways to improve the properties of metal materials. 2.3.1 Cooling curve and supercooling degree The cooling process of liquid metal can be measured by thermal analysis method to determine its temperature change pattern. Put the molten metal liquid into a container that dissipates heat slowly, and let the metal liquid cool down extremely slowly. During the process, the temperature is measured at regular intervals and the temperature-time change curve is drawn, that is, the cooling curve, as shown in Figure 2-12. It can be seen from the cooling curve that as the cooling time increases, the liquid metal continues to release heat and the temperature decreases. When the temperature decreases to point a, a crystallization temperature platform ab (i.e., crystallization stage) appears. This shows that during this time period, heat is released inside the metal to make up for the heat loss. The heat released during crystallization is called latent heat of crystallization. The temperature of the crystallization platform on the cooling curve is called the crystallization temperature T0. Theoretically, the crystallization temperature T0 is the melting point temperature of the metal, that is, the theoretical crystallization temperature. When all the liquid metal solidifies and is no longer released, the temperature drops over time. In actual production, the cooling of metal cannot be extremely slow, causing the actual crystallization temperature T1 to be lower than the theoretical crystallization temperature T0, as shown in Figure 2-13. The difference between the theoretical crystallization temperature and the actual crystallization temperature is called the degree of supercooling ΔT, and the phenomenon that the actual crystallization temperature is lower than the theoretical crystallization temperature is called supercooling. The degree of subcooling is related to the cooling rate of the metallic liquid. During crystallization, the greater the cooling rate, the greater the degree of supercooling. ； The greater the degree of supercooling, the greater the crystallization driving force and the faster the crystallization speed. 2.3.2 Metal crystallization process The metal crystallization process is a process in which crystal nuclei are continuously formed and the crystal nuclei continue to grow, as shown in Figure 2-14. (1) Nucleation During the process of cooling the liquid metal from high temperature to low temperature, there are small groups of atoms that come and go. These small groups of atoms are the source of subsequent crystal nuclei, which are called crystal embryos. As the temperature decreases, the volume of the crystal embryo continues to increase. When the liquid is cooled below the crystallization temperature, the size of the crystal embryo reaches a certain limit and it can exist stably. This kind of crystal embryo that can exist stably is called a crystal nucleus, and this process is called a nucleation process. (2) Growing and stable crystal nuclei As time prolongs and the temperature drops, atoms in the liquid metal continue to accumulate on the surface of the crystal nuclei, and the crystal nuclei continue to grow. In the early stages of the growth of crystal nuclei, due to the regular arrangement of internal atoms, most of their shapes are relatively regular, as shown in Figure 2-15. But as the crystal nuclei grow and the edges of the crystal form, the heat dissipation conditions at the edges are better than other parts and they grow preferentially. They grow branches like tree branches, then branches, and finally fill the spaces between the crystals. When the liquid metal is consumed, the crystallization process ends. From the above analysis, it can be seen that generally pure metals are polycrystals composed of irregular-shaped grains and grain boundaries grown from many crystal nuclei. 2.3.3 Grain size and control (1) The influence of grain size on the mechanical properties of metals The size of grains is one of the important factors affecting the properties of metal materials. Generally speaking, the finer the grains, the higher the strength and hardness of the metal material, and the better the plasticity and toughness. This phenomenon is called fine-grain strengthening. Because as the grains refine, the more and more tortuous the grain boundaries are, the more opportunities there are for interlocking between grains, which is less conducive to the propagation and development of cracks. Therefore, how to control the size of grains in production is a very important issue. (2) Control of grain size. Any factors that can promote nucleation and inhibit growth can increase the number of crystallized grains and refine the grains. Therefore, there are several ways to refine the grains during crystallization:: ○1 Increase the degree of supercooling. The nucleation rate N (the number of crystal nuclei formed in a unit volume of liquid per unit time) and the growth rate G (the linear speed at which crystal nuclei grow per unit time) are both related to the degree of supercooling. They both increase as the degree of supercooling increases, as shown in Figure 2-16. However, the two rates of increase with the increase of supercooling are different, and the growth rate of nucleation rate is greater than the growth rate. Therefore, by increasing the degree of supercooling (above a certain value), a relatively fine grain structure can be obtained. Increasing the cooling rate of liquid metal is the main way to increase the degree of supercooling. Therefore, a relatively fine grain structure can be obtained by increasing the cooling rate of liquid metal. Measures such as lowering the pouring temperature of the metal solution and using metal molds instead of sand molds can increase the cooling rate of liquid metal. ○2. Modification treatment: Before the metal solution crystallizes, certain substances (called modifiers) are intentionally added to the metal solution to create a large number of artificial crystal nuclei, thereby increasing the number of crystal nuclei. * * Increase to achieve the purpose of refining the grains. This method of refining the grains is called metamorphism. For example, adding trace amounts of titanium or sodium salt to aluminum or aluminum alloys, and adding trace amounts of titanium or aluminum to steel are typical examples of modification treatment. ○3. When additionally vibrating the metal solution to crystallize, mechanical vibration, ultrasonic vibration or electromagnetic vibration can be used to move the metal liquid in the casting mold, so that the crystals are continuously broken during the growth process, and the broken crystal blocks act as crystal nuclei, and finally a fine grain structure is obtained. 2. 3.4 Structure of metal ingots (pieces) Metal parts used in industry are generally obtained in two ways.: One is formed by solidifying liquid metal directly in a casting mold of a certain geometric shape and size. This is called a casting. ； The other is to pour liquid metal into square or round ingots, then cut them out, and then go through hot rolling or hot forging. Finally, the geometric dimensions and properties of the parts may be obtained through machining and heat treatment, or even welding. For castings, the as-cast structure and defects directly affect its mechanical properties. ； For ingots, the as-cast structure and defects directly affect its processing performance and may also affect the mechanical properties of the final product. Therefore, the quality of alloy castings (or ingots) is important not only in casting production, but also for almost all metal products. The grains after metal solidification are relatively coarse and are usually visible macroscopically, as shown in Figure 2-17. (1) Surface fine-grained area When liquid metal is injected into the ingot mold, due to the low temperature of the mold wall, the thin layer of molten liquid in contact with the mold wall generates strong supercooling, and the mold wall can be used as a base for non-uniform nucleation. Therefore, a large number of crystal nuclei are immediately formed on the surface of the ingot mold. These crystal nuclei grow rapidly until they contact each other, forming a fine-grained area composed of small, densely oriented, and uniform equiaxed grains. (2) As the fine-grained area on the surface forms in the columnar crystal area, the mold wall is heated by the molten metal and continues to heat up, which slows down the cooling of the remaining molten metal. In addition, due to the latent heat released during crystallization, the supercooling of the liquid at the front of the fine-grained area decreases, and the nucleation rate decreases. As a result, the nucleation rate is not as high as the growth rate, and each crystal grain can grow faster. Since heat dissipation is fastest in the direction perpendicular to the mold wall, and grains in other directions are restricted by adjacent grains and cannot develop, therefore, the crystal preferentially grows in the direction perpendicular to the mold wall to form columnar crystal regions. (3) After the columnar crystals in the center coarse equiaxed crystal area grow to a certain extent, because the front melt moves away from the mold wall, the temperature difference in the melt decreases, and the heat dissipation has no obvious directionality and tends to a uniform cooling state. The molten metal at the center far away from the mold wall has difficulty in dissipating heat, the degree of supercooling is small, the nucleation rate decreases, and the crystal nuclei grow at a constant speed, so a coarse equiaxed crystal area in the center is formed. It can be seen from the above that the structure of the ingot is uneven. The fine-grained area on the surface of the ingot has a dense structure and good mechanical properties. However, this area is very thin and has little impact on the performance of the ingot. The structure of the columnar crystal area is denser than that of the coarse equiaxed crystal area in the center. Its plasticity is poor and anisotropic, and it is easy to crack during forging and rolling. Therefore, columnar crystal regions are undesirable for metals with poor plasticity and high melting points. In production, methods such as vibration pouring or modification treatment are often used to inhibit the expansion of the columnar crystal area during crystallization. However, because the mechanical properties of columnar grains in the length direction are better, directional solidification is often used to obtain columnar structures for non-ferrous metals (non-ferrous metals) and their alloys with good plasticity or parts that bear unidirectional loads, such as steam turbine blades. The coarse equiaxed crystal area in the center has a loose structure, more impurities, and lower mechanical properties. In metal ingots, in addition to uneven structures, defects such as shrinkage cavities, shrinkage porosity, bubbles and segregation often exist. These defects will also affect ingot (casting) quality and performance. Ingot defects are discussed in Chapter 9.